If you observe the error Headers were not valid when importing a spreadsheet into the web store, this usually indicates that:

1. There are either more columns of data than expected and/or
2. There is extraneous data outside of the main columns

Be sure your import matches the original export from the web store. Additionally, try deleting or clearing the contents in all columns outside the main columns and rows after the last entry, even if the columns/rows do not appear to have data.